We all want the best for our boats—but we also don’t want to be fiscally foolish. That’s why a lot of us struggle with the question of synthetic motor oil: Is it safe to use? Worth the extra money? Will we notice any benefit from it?
To sort out this quandary, begin at the beginning: What is synthetic oil? Well, it’s not conventional oil, which is crude that has been refined and to which various chemical compounds have been added to meet the criteria required for specific applications—things like viscosity, detergency, and resistance to oxidation. Synthetic oil starts with a base of synthesized chemicals, some of which may be derived from petroleum, to which more chemical compounds are added.
Eliminating crude means eliminating naturally occurring contaminants like wax, but more importantly, it means the oil molecules are smaller and of more uniform size so they produce a more even film that does a better job of keeping metal components from touching each other. Synthetics have more sophisticated additives that provide stable viscosity across a wider temperature range and reduce friction, especially at startup.
All this has its price. Synthetics are at least twice as expensive as conventionals. What do you get in exchange? Less wear, mainly because of more stable viscosity and resistance to oxidation, high-temperature breakdown, and sludging. How much less? Depends on who you talk to: The answers I got ranged from “insignificant” to “double the life.” In truth, I could find no definitive studies that quantified the improvement, but everyone I spoke to agreed that unless you routinely run engines to the point of rebuild, this benefit will be of limited value.
A bigger advantage is extended oil life. The consensus is that under normal operation an automotive engine running synthetics can go 15,000 miles between oil changes, roughly double manufacturers’ normal recommendations. If synthetic oil costs double but you change it half as often, it’s a financial wash—plus you get lower wear, better cold starts, and (at least according to some) increased fuel efficiency due to less friction. But if you typically change your oil not according to the amount of accumulated hours or miles but rather by the calendar, as many boaters do, you may not see any benefit here either.
Price is a big obstacle for synthetics, which is why retailers invented “partial synthetics,” typically 30-percent synthetic, 70-percent conventional. (Synthetic content can be as little as 10 percent for oil to qualify in this category.) While this formulation does drop the price, logic says it also reduces the advantages of full synthetics, and I could find no consensus as to how much a synthetic mix will allow you to reduce your oil-change interval. The very existence of these oils puts to rest the myth that you cannot switch between types.
Should you use synthetic oil in your boat engines? Because they operate under such heavy loads, marine engines would seem to be a perfect fit for them, but again, much turns on how often you use your boat and how long you plan to keep it. If you’re not going to wear out your engines, you’re not really going to benefit from lower wear. And if like most boaters you routinely change your oil each year, extended oil-change intervals don’t really apply. You can probably do just fine with a good-quality conventional oil.
But that hasn’t stopped engine manufacturers from jumping on the synthetic bandwagon. Default recommendations seem to be synthetic—usually one branded by the manufacturer. MerCruiser recommends its own full-synthetic oil for use in its gasoline engines, then lists acceptable alternatives (in order of preference): MerCruiser’s synthetic blend, MerCruiser’s conventional oil, and “other recognized brands” of four-cycle oil.
Of 20 service yards I contacted for this article, all but two said they use synthetics in oil changes unless the customer deems otherwise. Clearly capitalism is at work here; synthetics cost more, and so the profit margins on them are higher. Engine manufacturers claim their branded synthetics are superior. Unless you’re a chemist, you may just have to trust them on this because the alternatives are few. If you have gasoline engines—inboard or outboard—FC-W is the rating you want awarded by the National Marine Manufacturers Association.
With diesels the issue is cloudier. Theoretically you can choose a synthetic oil carrying the American Petroleum Institute’s (API) highest rating: CJ-4, designated for “Tier 4 nonroad exhaust emissions standards as well as previous year model diesel engines.”
The safest course of action is, of course, to use the synthetic oil your engine manufacturer recommends, even if it costs more, doubly true if your engine is under warranty. But synthetic oils need not be your default choice. As long as your engine is out of warranty, you may get perfectly adequate protection from conventional oil, as long as you choose the right one and follow proper change recommendations. Is it worth paying extra money for the added benefits synthetics offer? You’ll have to decide for yourself.